1. Field of the Invention
The present invention generally relates to light-emitting devices, and particularly to the structure and manufacture of a light-emitting device having high-brightness characteristic.
2. Description of the Related Art
A light-emitting diode is conventionally composed of a multi-layer structure including active layers sandwiched between n-type and p-type semiconductor layers. The active layers are configured to irradiate light in response to an electric signal applied between electrodes of the light-emitting diode. The electric stimulation creates an injection of electrons and holes from the n-type and p-type semiconductor layers into the active layers where they recombine to produce light.
Conventionally, the efficiency of the light-emitting diode can be characterized through a number of indicative factors. One factor is the light extraction efficiency, which is the ratio of the amount of light leaving the light-emitting diode relative to the amount of light produced in the light-emitting diode. Practically, the amount of light leaving the light-emitting diode is less than the amount of light produced in the light-emitting diode due to diverse inner absorption paths. To increase the light extraction efficiency, reflectors are conventionally placed inside the multi-layer structure of the light-emitting diode to redirect light into useful directions.
To address the foregoing issue, one approach known in the art consists of forming a p-type electrode made of silver (Ag) on the p-type layer of the light-emitting diode. This technique is described in, for example, U.S. Pat. No. 6,194,743, the disclosure of which is incorporated herein by reference. The high reflectance of Ag contributes to form a reflective p-type electrode capable of redirecting light towards the substrate, and absorption through the p-type electrode can be thereby prevented.
Another approach known in the art is described in PCT application number PCT/US00/35184, the disclosure of which is incorporated herein by reference. This other approach proposes a multi-layer reflective contact structure that includes specific reflective metal layers selected to optimize the reflectance and electric characteristic of the contact.
Though the foregoing techniques contribute to enhance the light extraction of the light-emitting device, but experiment tests show that the reflective metallic contact may be the cause of an unstable thermal behavior of the light-emitting diode. For example, if the operating voltage of the light-emitting diode is initially set at 3.3V, assembling the light-emitting diode on a substrate via a reflow process at a temperature of 250° C. will bias the operating voltage to 4.2V. Such a voltage deviation is undesirable. The device stability may be slightly improved with the association of indium tin oxide (ITO) with silver, but ITO has a poor adhesion in respect of metal.
Therefore, there is presently a need for a light-emitting device that can overcome the prior disadvantages and provide high-brightness characteristic.